If the bow jumps around a lot when you play a note, it's probably too tight. If the tone is anemic and sickly sounding, it's probably too loose. After a while you will get the feel of just how tight you want the bow to be. This is a very personal decision. Some people like a tight bow, others a more slack one. The hair should not touch the stick when you are playing loudly. If the bow is not grabbing, no matter how hard you press, there is not enough rosin on it. You should be able to get the string to speak with reasonable ease. Be sure to wipe off the rosin from the instrument when you finish playing. You don't want it to build up and make the top sticky.